A rapid increase in use of fossil fuels has led to an increase in the demand for alternative energy or clean energy. As a part of such demand, most actively investigated fields are power generation/storage applications based on electrochemistry.
At present, a representative example of electrochemical devices using the foregoing electrochemical energy may be a secondary battery, and the application range thereof continues to expand.
In recent years, increased technological development and demand for mobile equipment such as a portable (laptop) computer, a mobile phone, a camera, etc. have led to a rapid increase in the demand for secondary batteries as an energy source. Among these secondary batteries, lithium secondary batteries having high energy density and operational (output) voltage, long cycle life and low self-discharge ratio are extensively studied, commercially available and widely used.
In addition, increased concern over environmental issues has brought about a great deal of research associated with electric vehicles (EV) and hybrid electric vehicles (HEV) as substitutes for vehicles using fossil fuels, such as gasoline vehicles and diesel vehicles, which are a major cause of air pollution. Although nickel metal hydride (NiMH) secondary batteries have generally been used as a power source of such EVs and/or HEVs, a great deal of studies into use of lithium secondary batteries having high energy density and high discharge voltage are underway and some of these are commercially available.
A typical lithium ion secondary battery uses graphite as an anode active material, wherein lithium ions in a cathode are inserted into and detached from the anode, thus repeating a charging and discharging process. Although the theoretical capacity of a battery may be varied according to kinds of electrode active materials, it is generally known that charge-discharge capacity is deteriorated as (charge-discharge) cycles are repeated.
The major reason behind such a problem is considered to be that electrode active materials are mutually separated or an electrode active material is isolated from a current collector, due to variation in volume of an electrode caused by repeatedly charging and discharging a battery over time, and the active material does not fully function. In addition, due to lithium ions entering the anode during insertion and detachment completely escaped therefrom, an active point of the anode is lowered which in turn may deteriorate charge-discharge capacity and lifespan while the cycles are repeated.
Especially, in order to increase discharge capacity, in the case where some materials such as silicon, tin or a silicon-tin alloy, are combined with natural graphite having the theoretical discharge capacity of 372 mAh/g for use, volume expansion of the material may be considerably increased during repeated charge and discharge. Due to this, an anode material is released from the electrode material and causes a problem of sharply deteriorating battery capacity over the course of repeated charge-discharge cycles.
Accordingly, studies for binders and/or electrode materials that have strong adhesiveness to prevent mutual separation of electrode active materials or separation of an electrode material from a current collector in the manufacture of an electrode, possess excellent physical properties to control volume expansion of the electrode active material occurring during repeated charge-discharge, to thereby contribute to a structural stability of the electrode as well as improvement of battery performance, are strongly required in the related art.
Since the existing solvent binder, i.e., polyvinylidene fluoride (PVdF) does not satisfy the above requirements, a method of preparing emulsified particles by polymerization of styrene-butadiene rubber (SBR) in an aqueous phase and mixing the prepared particles with a neutralizing agent and using the same has recently been proposed and becomes commercially available. Such a binder has advantages of being eco-friendly and reducing the content of the binder to increase battery capacity. However, although adhesion durability may be improved by elasticity of a rubber fraction, adhesiveness is not remarkably superior.
Therefore, there is still a strong need for development of a novel binder capable of providing structural stability to an electrode while improving cycle characteristics of a battery, as well as having excellent adhesiveness.